1. Technical Field of the Invention
The present invention relates generally to the mobile telecommunications field and, in particular, to a method for processing multiple random access mobile-originated calls.
2. Description of Related Art
The next generation of mobile communications systems will be required to provide a broad selection of telecommunications services including digital voice, video and data in packet and channel circuit-switched modes. As a result, the number of calls being made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the new generation of mobile communications systems will have to use much faster and flexible random access procedures, in order to increase their access success rates and reduce their access request processing times.
In most mobile communications systems, such as, for example, the European joint development referred to as the xe2x80x9cCode Division Testbedxe2x80x9d (CODIT), a mobile station can gain access to a base station by first determining that the RACH is available for use. Then, the mobile station transmits a series of access request preambles (e.g., single 1023 chip symbols) with increasing power levels, until the base station detects the access request. In response, the base station starts the process of controlling the mobile station""s transmitted power via a downlink channel. Once the initial xe2x80x9chandshakingxe2x80x9d between the mobile station and base station has been completed, the mobile user transmits a random access message.
More specifically, in a CODIT-based Code Division Multiple Access (CDMA) system, a mobile station will attempt to access the base station receiver by using a xe2x80x9cpower rampingxe2x80x9d process that increases the power level of each successive transmitted preamble symbol. As soon as an access request preamble is detected, the base station activates a closed loop power control circuit, which functions to control the mobile station""s transmitted power level in order to keep the received signal power from the mobile station at a desired level. The mobile station then transmits its specific access request data. The base station""s receiver xe2x80x9cdespreadsxe2x80x9d the received (spread spectrum) signals using a matched filter, and diversity-combines the despread signals to take advantage of antenna diversity.
In an IS-95 CDMA system, a similar random access technique is used. However, the primary difference between the CODIT and IS-95 process is that the IS-95 mobile station transmits a complete random access packet instead of just the preamble. If the base station does not acknowledge the access request, the IS-95 mobile station re-transmits the access request packet at a higher power level. This process continues until the base station acknowledges the access request.
In a mobile communications system using a slotted ALOHA (S-ALOHA) random access scheme, such as the method disclosed in the above-described U.S. patent application Ser. No. 08/733,501 (hereinafter, xe2x80x9cthe ""501 Applicationxe2x80x9d), a mobile station generates and transmits a random access packet. A diagram that illustrates a frame structure for such a random access packet is shown in FIG. 1. The random access packet (xe2x80x9caccess request data framexe2x80x9d) comprises a preamble and a data field portion. The preamble contains a unique signature (bit) pattern, which is xe2x80x9cLxe2x80x9d symbols long. The signature pattern is randomly selected from a set of patterns that are, but not necessarily, orthogonal to each other. Note that although the random access request packet shown in FIG. 1 is described herein as including a preamble with a signature field, this description is exemplary and for illustrative purposes only, but not intended as a limitation. As such, the unique signature patterns can be transmitted in a different way (e.g., not within a preamble, integrated into the control channel, in parallel with a data field, etc.). As such, the use of this unique signature pattern feature, as described and claimed in the ""501 Application, provides a significantly higher throughput efficiency than prior random access schemes.
As described in the ""501 Application, the data field of the random access packet includes certain random access information, including mobile (user) identity information, required service number (number of services to be provided), required air time (time needed to complete a message), short packet data message (to increase transmission efficiency), and an error detection redundancy field (cyclic redundancy code). For reasons elaborated in the ""501 Application, the spreading ratio (spread spectrum modulation) of the preamble is selected to be longer than the spreading ratio of the data field portion. However, situations may be envisioned in which this is not necessarily so.
The random access packet (e.g., such as the packet shown in FIG. 1) is transmitted by the mobile station at the beginning of the next available slot. A block diagram of an apparatus that can be used in a mobile station to generate and transmit the random access packet illustrated in FIG. 1 is shown in FIG. 2. Essentially, as illustrated by FIG. 2, the preamble and data field of the random access packet are generated and spread separately (with respective spreading codes) and then multiplexed and transmitted by the mobile station.
Next, the random access packet transmitted by the mobile station is received and demodulated at the target base station with a matched filter-based receiver. FIG. 3 is a block diagram of a detection section (for one antenna) of a base station""s random access receiver, which functions primarily to estimate the timing of the received signal rays. The matched filter, which is used only during the preamble period, is tuned to the preamble""s spreading code. The matched filter is used to detect the presence of the random access request, and despread the preamble part of the random access packet and feed it to the accumulator unit. The accumulator (signatures 1-l) is a unique feature used for the ""501 Application""s random access method to sum the signals at the output of the matched filter during the preamble""s (M) symbol periods, in order to increase the received signal-to-interference (S/I) power ratio. Since each received preamble comprises a unique signature pattern, the accumulation operation is carried out with a plurality of accumulators (1-l), with each accumulator tuned to one of the possible signature patterns to be received.
FIG. 4 is a simple block diagram of an accumulator that can be used for the I channel (quadrature detection) in the random access detector section shown in FIG. 3. A similar accumulator can be used for the Q channel. Referring to FIGS. 3 and 4, the output of each accumulator (signature 1-l) is coupled to a peak detection unit. At the end of the preamble period, each peak detection unit searches the output of its respective matched filter for each signal peak that exceeds a predetermined detection threshold. Each peak detection unit then registers (detects and stores) the magnitude and relative phase of each of those peak signals, and thereby determines the number of significant signal rays available for demodulation in the receiver. As such, the timing of each peak is estimated and used to set the receiver""s xe2x80x9cRakexe2x80x9d parameters (Rake receiver sections 1-l). FIG. 5 is a block diagram of a random access demodulator that can be used to demodulate the data field portion of the random access packet. Essentially, the random access demodulator section decodes the data information in the received data field and checks for transmission errors.
Notably, although the random access apparatus and method described above with respect to FIGS. 1-5 has numerous advantages over prior random access schemes, a number of problems still exist that remain to be solved. For example, a large number of packet collisions may occur if mobile stations in all of the cells use the same. spreading codes during the preamble or data field processing stage. As a consequence, an excessive number of the random access requests will have to be re-transmitted, which can lead to system instability. Moreover, using the random access apparatus and method described above, since the random access requests are transmitted at the beginning of the next time slot, the base station""s matched filter receiver is not utilized as efficiently as it can be, because the matched filter receiver is idle for the complete period subsequent to the preamble reception stage. Additionally, since the length of the random access packet used with the above-described scheme is fixed, the size of the short data packets is restricted by the extent of use of the remainder of the packet. For all of these reasons, a more flexible random access request procedure is needed to resolve these problems. As described below with respect to FIGS. 6-8, U.S. patent application Ser. No. 08/847,655 (hereinafter, the xe2x80x9c""655 Applicationxe2x80x9d) successfully resolves the above-described problems.
Nevertheless, other random access problems still exist that need to be solved. For example, FIG. 9 is a time sequence diagram that illustrates how two or more random access requests can collide when they arrive simultaneously at a base station receiver. In order to minimize the number of collisions occurring during re-transmissions, the mobile station""s time-out period (time elapsed before re-transmitting a request) can be randomly selected from within a relatively long time interval {0, Td}, where Td is the maximum allowable time delay. The use of a relatively long time interval between re-transmissions reduces the probability of collisions. However, the average delay encountered waiting for a successful random access transmission can be rather long. Furthermore, although use of the random access methods in the above-described patent applications are highly advantageous over prior methods, random access collisions can still occur when two or more random access requests that contain the same signature pattern arrive simultaneously at the base station receiver. Nevertheless, as described in detail below with respect to FIG. 10, the present invention successfully resolves these and other related random access problems.
It is, therefore, an object of the present invention to utilize random access channels more efficiently.
It is another object of the present invention to be capable of receiving a significantly higher number of random access requests per matched filter than received by conventional means.
It is yet another object of the present invention to reduce the probability of collisions between random access requests and also minimize their loss.
It is still another object of the present invention to be capable of selecting the length of a data field in a random access request packet to allow increased flexibility in selecting the length of a short packet field.
It is yet another object of the present invention to provide a random access packet that can be used to quickly establish long data or voice calls.
It is still another object of the present invention to maintain a low level of cross-correlation between random access attempts made from neighboring cells.
Still another object of the present invention is to significantly shorten random access delays caused by random access request collisions or erroneous random access request arrivals at base station receivers.
Yet another object of the present invention is to significantly reduce the time interval between random access re-transmissions and thereby increase throughput efficiency.
In accordance with the invention described and claimed in the ""655 Application, the foregoing and other objects are achieved by a method that assigns each cell a unique preamble spreading code and a unique long-code which is concatenated with a short spreading code associated with a randomly selected signature, and is used to spread the data part of a random access packet. The period selected for the long-code can be relatively long in duration (e.g., up to hours or days in length). Also, the widths of the transmission time slots are set equal to the length of the preambles. Consequently, the mobile station""s random access requests can be timed to start at the beginning of the slots, and detected during the preamble periods by the matched filter in the base station""s random access receiver. The data field of the mobile station""s random access request is transmitted in the slots following the preamble and received by the rake receiver at the base station. However, subsequent to the preamble period, the matched filter is still enabled to receive the preambles of other random access requests. Therefore, the matched filter can be utilized continuously and more efficiently, and a significantly larger number of random access requests can be processed in comparison with prior random access schemes. As such, the communications throughput and efficiency of a random access system using the present method are substantially higher than the throughput and efficiency of prior random access systems.
Additionally, the length of the data field is not restricted. The method of concatenated spreading of the data field portion of the random access packet allows a user to generate a packet which is as long as desired. Moreover, the concatenated spreading removes the danger that the resulting packet will collide with other random access request packets, since the spreading pattern and/or its phase are unique.
In accordance with the present invention, the foregoing and other objects are achieved by a method that randomly selects new signatures for random access re-transmissions when collisions have occurred. As such, the present invention randomizes the re-transmission of random access requests over a signature domain instead of just over the time domain. Consequently, the present invention significantly shortens the random access delays caused by collisions or erroneous arrivals of random access requests at base station receivers, and also significantly reduces the time interval between random access re-transmissions. The present invention thus increases random access system throughput efficiency in comparison to prior random access approaches.